Acetylene solutions



Patented Aug. 13, 1946 UNITED STATES PATENT OFFICE ACETYLENE SOLUTIONSNo Drawing. Original application October 15, 1940, Serial N0. 361,274.Divided and this application May 11, 1943, Serial No. 486,560

3 Claims. 1

This invention relates to the absorption of acetylene. Moreparticularly, it relates to a process for the absorption and recovery ofacetylene by a cryogenic method. This application is a division of ourco-pending application Serial No. 361,274, filed October 15, 1940, nowPatent No. 2,383,547.

Numerous inventors have disclosed selective solvents for acetyleneadapted to the separation of acetylene from mixtures with other gases,particularly pyrolysis gases, for its recovery and enrichment. It hasbeen shown that acetylene may be absorbed in liquids such as acetone inunexpectedly high concentrations and successively recovered by degassingat elevated temperature and reduced pressure. These processes have beencommercially applied for the storage of acetylene and for its recoveryfrom dilute mixture in other gases, but their application is limited bysolvent losses and by the cost of power necessary to carry out thetemperature or pressure cycle.

Acetylene can be produced cheaply by pyrolysis of other hydrocarbonmaterials either thermally or by electric processes, but, in so doing,it is also accompanied by relatively large volume of byproduct gases,such as hydrogen, ethane, ethylene, etc.

The objects of thi invention are to devise new processes for theseparation of acetylene from In order that the process may be more fullyunderstood, the following specific examples are given. Such examples aremerely by way of iilustration. The invention is not limited thereto, butsuitable variations may be made as will become more apparenthereinafter.

Example I One hundred (100) volumes of dioxane was cooled to atemperature slightly above its freezing point (approximately 10 C.) in aclosed vessel equipped with a gas inlet tube and vigorous agitation.Acetylene at about 760 mm. pressure was passed into the dioxane whilestirred until saturated. Measurement of the acetylene absorbed showedthat approximately 2400 volumes was in the cooled dioxane solution. Ifdesired, the acetylene may be stored in the form of this solution. Onfreezing this dioxane solution with agitation to prevent entrapping thegas, 2400 volumes of acetylene was recovered.

Example II Impure acetylene was passed up through an absorption towercounter-current to a stream of dioxane at about 10 C. and 760 mm.pressure and at a rate of 20 volumes of gas per volume of solvent. Theinput gas had the following composition (parts by volume) such gases andfor its storage. A further object Per cent is to devise a method ofseparation and storage 02H? 12.9 in which the cycle can be operated withminimum C2H4 19.5 loss of solvents and low power consumption. 18.8 Otherobjects will appear herein below. H8 45.3

These objects have been accomplished by the er gases 3,5

discovery of a new acetylene absorption cycle which involves absorbingthe acetylene in a se- After contact with the solvent, the eflluent gashad the following composition:

lective solvent and then freezing the resulting Per cent solution,whereby the acetylene is recovered in a can; 1.1 gaseous form. This willbe referred to as the 40 (321-14 5, cryogenic method- CzHa 22.2

In accord with the prior art, acetylene is ab- H2 53, orbed in numeroussuitable solvents, the caot 4,2

pacity of the solvent increasing with decrease in temperature. We havediscovered the unexpected fact, however, that, as the temperature isfarther lowered, the acetylene is suddenly disengaged and released fromsolution, with the accompanying freezing of the solvent. We have Thesolvent leaving the absorption tower was passed into the second vesselin which it was frozen by lowering its temperature. The composition ofthe disengaged gas from the solvent was approximately practicallyutilized this unexpected discovery for CzHz Per 9 9 the storage andseparation of acetylene by the (32H, :IZIIII: simple procedure ofabsorbing acetylene in the solvent at some temperature above itsfreezing Example point and subsequently liberating the acetylene Theenriched acetylene obtained in the process from solution by freezing. ofExample II was again contacted with dioxane in an absorption column inthe manner of Example II. In this case, the eiiiuent gas from theabsorption tower was essentially ethylene and, upon freezing thesolvent, the disengaged gas had the composition Per cent CaHa 94.8Cal-I4 5.2

Example IV The process of Example I was repeated, using a temperature of5 C. for the absorption of the acetylene. Three thousand (3000) volumesof acetylene was absorbed in the 100 volumes of solvent and waspractically quantitatively recovered on freezing. This illustrates how,by virtue of the lowering of the freezing point of the dioxane solutionby the dissolved acetylene, it is possible to work at a temperaturesomewhat below the freezing point of the pure solvent and thus takeadvantage of the greater solubility at this temperature. By working at apressure of 1040 mm., 4300 volumes of acetylene was absorbed andrecovered.

The dioxane used in Example I may, of course, be replaced by othersolvents, particularly those of the type more fully describedhereinafter. The following table gives the volumes of acetylene atatmospheric pressure absorbed by 100 volumes of several other typicalsolvents together with the temperature used.

The process of Example II may be made completely continuous as follows:The solution which has been in contact with the gas is removedcontinuously from the bottom of the absorption tower and passed into avessel, in which a drum which is internally cooled to a temperatureseveral degrees below the melting point of the solution (for example, 0C. in the case of dioxane), rotates about a horizontal axis in such away as to be only partly immersed in the solution. A layer of solutionin contact with this drum is thus caused to freeze with the liberationof the dissolved gas. The frozen layer of the solvent upon the drum isthus removed from the solution and is scraped ofi by a knife, melted,and returned to the top of the absorption tower. At the same time, theliberated gas is continuously removed. By adjusting the size and rate ofrotation and temperature of the drum, the liberated gas and recoveredpure solvent may be formed at a rate corresponding to the removal of theacetylene solution from the absorption tower.

Although any solvent capable of dissolving acetylene will function, thebest results are ob-- tained with those which have a high absorptivecapacity in the liquid state, a very small absorptive capacity in thesolid state, a freezing point in a suitable range as is more fullydiscussed below, and a high selective action toward acetylene. We havefound that those solvents best suited for the process contain oxygen ornitrogen or both and contain one or more atoms which are capable ofreadily losing electrons or, in other words, atoms which areelectron-donor atoms. Furthermore, these compounds should, preferably,contain no labile hydrogen more acidic than the hydrogen of acetyleneand should have a closely knit crystalline lattice in the solid state.Heterocyclic organic compounds containing, in the ring, at least twoatoms of the group consisting of oxygen and nitrogen, constitute a quiteuseful type of solvent. Among the solvents possessing thecharacteristics suitable for acetylene absorption, compounds of thefollowing types may be mentioned:

Glycol carbonate Ethylene derivative of N-methyl carbamic acid Ethylenederivative of N,N'-dimethyl urea, Glycol oxalate Ethylene derivative ofN,N'-dimethyl oxamide Ethylene derivative of N-methyl oxamic acid Glycolether ester of glycolic acid Ethylene derivative of glycolic acidN-methyl amide Dioxane N-acetyl morpholine N-formyl morpholineN-carbomethoxy morpholine N-carboethoxy morpholine N-carbopropoxymorpholine N-methyl morpholine N,N'-dimethyl pyrimidine DiglycolideN-methyl succinimide Acetonyl acetone Furfuryl acetone AcetophenoneGamma-pyrone Dimethyl carbonate N-dimethyl carbamic acid methyl esterTetramethyl urea Dimethyl oxalate N-dimethyl oxamic acid methyl esterTetramethyl oxamide N,N'-dimethyl amide of glycolic acid methyl etherDimethyl acetamide Glycol diacetate Paraldehyde Ethylene glycol acetalEthylidene ether ester of glycolic acid Anethole Safrole The term"ethylene derivative is used above to describe compounds in which thedivalent radical CH2-CH2- replaces 2 hydrogen atoms attached to oxygenor nitrogen of the parent substance. Thus, the ethylene derivative of N-methyl carbamic acid has the structure and is systematically known as1-methyl-2- oxazolidone. Similarly, the ethylene derivative ofN,N'-dimethyl urea has the structure OHs-N-C ON(CH:)CH:CH;

and is known as 1,3-dimethyl-2-imid azolidone, and the derivative ofN,N-dimethyl oxamide, which has the structure and is known as1,4-dimethyl-2,3-piperazine dione. Also included among the suitablesolvents are homologues of the compounds given in the above list. Thus,by the use of appropriate homologues, it is often possible to alter thephysical properties such as the melting point in accordance with therequirements discussed below without substantial change in absorptivecapacity. Mixtures of solvents may also be used for the same purpose.

Of the solvents mentioned above, dioxane is particularly adapted for theprocess of this invention.

Since the feature of this invention is the re-' covery of acetylene byfreezing the solvent after absorption, it is desirable to have a solventpossessing a freezing point in a convenient temperature range. As inother processes of the known art, the capacity of the solvents for theabsorption of acetylene decreases with increase in temperature;therefore, though it is within the scope of the invention, it is notparticularly desirable to have solvents with too high freezing points.On the other hand, though the capacity of the solvent is greatlyincreased, at greatly reduced temperature, the cost of refrigerationrequired for freezing makes it undesirable to have solvents freezingsubstantially below room temperature. A preferred range, therefore, aresolvents freezing between 10 C. and +50 C.

The temperature range in which the process of this invention may becarried out is not critical. As has been stated, the capacity ofsolvents is dependent upon the temperature, but the process is operablewithin almost any range. Obviously, since the capacity increases withdecrease in temperature, it is desirable to carry out the absorption ator slightly above the freezing point of the solvent.

Since the capacity of the solvent is increased by increase in pressure,it may be desirable under suitable conditions to absorb at pressureexceeding atmospheric within the limits of safety in handling acetylene.The pressure of absorption is not critical, and the upper limit ofpressure is restricted only by safety. On the other hand, degassing ismore rapid and more complete at low pressure. The practical method .ofoperation, therefore, is to absorb acetylene at super-atmosphericpressure and degas by freezing at'atmospheric or even sub-atmosphericpressure.

There are many advantages of this new discovery. First, it is possibleto carry out the absorption and recovery cycle with low power costs, inview of the fact that a proper selection of solvent permits theabsorption and freezing within ordinary temperature range where thethermal cycle can be supplied simply with water cooling. Second, sincethe disengagement upon freezing is practically quantitative, it ispossible to carry out the cycle at ordinary pressures without costlycompression and obtain high product eiliciency. Third, since thedegassing cycle occurs at the freezing point of the solvent and may becompleted with the solvent entirely in the solid phase. the vaporpressure of the solvents is low and solvent lossesare minimized.Manywother advantages will be apparent to one experienced in the art.

The composition of the acetylene-containing gas is immaterial. As inprocesses of the prior art, solvents suitable for this inventionselectively absorb acetylenic compounds and to a lesser degree otherunsaturated compounds. Typical mixtures of acetylene with hydrogen andother hydrocarbon gases such as might be produced in a pyrolysis or arcacetylene processes may be practically employed. Obviously, it may beapplied to the storage of pure acetylene.

It is apparent that many widely different-embodiments of this inventionmay be made without departing from the spirit and scope thereof, and,therefore, it is not intended to be limited except as indicated in theappended claims.

We claim:

1. A composition of matter consisting of a solution of acetylene in an N-substituted morpholine in which the substituent on the N atom of themorpholine molecule is of the group consisting of CH:, COH, COCH:, and

CO-O(CH:) 3-H, wherein n stands for a number from 1 to 3.

2. A composition of matter consisting of a solution of acetylene informyl morpholine. 3. A composition of matter .consisting of a solutionof acetylene in acetyl morpholine.

WILLIAMEHAIELL. RICHARDRVOGT.

